The primate visual cortex is the end station for the reception, integration and perception of visual impulses. The complex functional organization into vertical columns and horizontal hierarchical laminae has been well documented, yet little is known about the ultrastructural correlates to these functional properties, nor about the intricate interconnections within the cortex as well as with other visual centers of the brain. It is the aim of this study to examine in detail both the intrinsic synaptic organization of the visual cortex as well as its extrinsic connections with other cortical and subcortical areas through the use of modern neuroanatomical techniques. Extensive sets of serial thin sections will be attempted in order to trace and analyze synaptic interplay between the many neurons and neuronal processes, both arising from local as well as distant sources. The injection of histochemical and radioactively-labelled tracer substances will be utilized to examine intrinsic and extrinsic, afferent and efferent connections of the visual cortex. Such techniques are based on the physiological process of axoplasmic transport and clearly surpass the traditional lesion and degeneration techniques in their precision and reliability. The developmental characteristics of neurons and synapses within the visual cortex of prenatal and early postnatal monkeys will also be examined with the above techniques in the hope that the simpler system may provide insight into the mechanism of structural and functional integration in the formation of the adult visual cortex. Knowledge of neuronal and synaptic connections is crucial for the eventual understanding of functional circuitries within the visual system, and within the sensory systems in general.